1. Field of the Invention
This invention relates to curable organosiloxane compositions. More particularly, this invention relates to relatively low viscosity organosiloxane compositions that yield tough gels and elastomers upon curing.
2. Background Information
Elastomeric silicone materials, particularly those containing fillers such as finely divided forms of reinforcing silica and quartz, exhibit high levels of physical properties, including tensile strength, modulus and tear strength, that make these materials desirable in a variety of end use applications that utilize these properties.
Toughness is desirable for gels and elastomers that are used as coating, potting and encapsulating materials in electrical and electronic applications. Toughness can be defined as a combination of tensile strength, tear resistance and the ability to be elongated without ripping or tearing, which has been referred to as flexibility. This property is particularly desirable for materials that in order to function effectively must retain their integrity without cracking or crazing during repeated cycles of thermally induced expansion and contraction.
In many instances elastomer and gels used as coating, potting, and encapsulating materials must maintain adhesion to electronic and electrical components and printed circuit boards in addition to the electrical connectors and conductors that pass through the coating or encapsulating material. When the coating material expands, the connectors and conductors can act as initiation points for tears or cracks in the absence of sufficient toughness to dissipate the stresses occurring during expansion and contraction of the materials.
Organosiloxane elastomers and gels exhibiting sufficient toughness for electronic applications are typically prepared from compositions containing high molecular weight polyorganosiloxanes and/or high loadings of reinforcing fillers. The viscosity of these prior compositions is typically too high for use in coating and encapsulating intricately shaped substrates unless the compositions are diluted with solvents or vacuum assisted techniques such as vacuum impregnation are employed.
U.S. Pat. No. 5,145,933, which issued to Grisoni et al. on Sep. 8, 1992 describes organosiloxane gel forming compositions that cure by a platinum-catalyzed hydrosilation reaction. The curable compositions contain a polyorganosiloxane with vinyl radicals located only on non-terminal silicon atoms and two different types of organohydrogensiloxanes. One is a polydiorganosiloxane (1) containing silicon-bonded hydrogen atoms only at the terminal silicon atoms, and the second polyorganosiloxane (2) contains an average of at least three silicon-bonded hydrogen atoms per molecule, at least a portion of which are located on non-terminal silicon atoms. The percentage of the total silicon bonded hydrogen contributed by polyorganosiloxane (1) is equal to 81.36 minus a factor equal to 3.6 times the molar ratio of all silicon bonded hydrogen atoms in the curable composition, irrespective of source, to all of the alkenyl radicals present in the composition.
U.S. Pat. No. 4,340,709, issued on Jul. 20, 1982 to Jeram et al. relates to organosiloxane compositions that cure by a platinum-catalyzed hydrosilation reaction and contain the two types of organohydrogensiloxanes described in the Grisoni et al. patent. The molar ratio of the vinyl radicals present in the curable polyorganosiloxane to the silicon bonded hydrogen atoms present in the organohydrogensiiloxane (1), referred to by Jeram et al. as a hydride coupler, is from 0.75 to 1.5. While Jeram et al teach that the viscosity of the vinyl-terminated curable polyorganosiloxane can be from 100 to 200,000 centipoise (0.1 to 200 Pa.s) the lowest viscosity disclosed in the exemplified compositions (examples 2 and 3) is a mixture of 61.8 parts of a dimethylvinylsiloxy-terminated polydimethylsiloxane with a viscosity of 4,000 cps (4 Pa.s) and 5 parts of a dimethylvinylsiloxy-terminated polydimethylsiloxane with a viscosity of 0.5 Pa.s. In these compositions the hydride coupler contributes 70 and 74 percent, respectively, of the silicon-bonded hydrogen atoms present in the curable compositions, and the compositions contain a reinforcing silica filler. The curable compositions of example 2 and 3 had mixed viscosities of 40 and 85 Pa.s, respectively.
One objective of this invention is to provide curable organosiloxane compositions that are sufficiently low in viscosity to be applied to substrates in the absence of solvents or diluents using conventional coating and encapsulating techniques, yet which cure to yield tough elastomers and gels.